So, TEPCO was set to start removing spent fuel rods from a damaged cooling pool atop reactor number 4. Again, reactor number 4 had no problems, but complications with piping etc. connected to this cooling pool (for rods that were used well before the accident) caused the pool (which happens to sit atop reactor 4) to malfunction and lose water.

This is to be a very delicate procedure. Depending on what source you believe, it could be not so scary (unless you are one of the people at the plant) to apocalyptic if something goes wrong.

This article sums up both sides of that

After many organizations and gov'ts said they wanted to provide more advice and assistance before TEPCO took the plunge, TEPCO decided to do more testing and has delayed beginning the very sensitive (and long) removal process for two weeks.
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We are what we repeatedly do - Aristotle